I read that 10 armor is about equal to 1 all resist, this was in patch 1.0.3.
Now at pacth 1.0.5.....

Is this ratio still true or what do you use?

its a thumb rules and vary if you dont have any resist and tons of armor then resist will gain more value then 1:10 and vice verca
and you want about 100 all resist for each armor for both to gain their efficient value i believe.

so if you have 5000armor:500 resist it should be somewhat true
4000armor:500 resist then +10 armor should be > 1 resist and vice verca

It's a rule of thumb, but like every rule of thumb there are many exceptions and caveats. For example, while 10:1 is true for my CMWW wizard and my barb, it's rather 5:1 for my Archon wizard because I've been neglecting armor while getting more allres. There are two interpretations of the 10:1 rule:

1) It's what you will gain for this stat (i.e., 10 armor will net you as much EHP as 1 allres)
2) It tells you what your final stats should look like.

I would treat both with care. For the first, I already told you an example of my wizard who is in desperate need of more armor because my recent purchases just came with a lot of allres and no armor. You can check what your current ratio is if you load your character into a spreadsheet, such as the d3up.com calculator that will show you "EHP gain per stat".

Secondly, I think the 10:1 rule as a goal isn't universally true. My personal opinion, totally based on my subjective experience and with no theorycrafting behind it, is that I like to have at least 500, if possible even 700+ allres for act 3 MP 2 Archon farming. Armor should be around 4000, but getting to 5000 or higher without Energy Armor is very difficult and imho too expensive. For a barbar, however, you should be fine with 700 allres or even less, but at the same time you gain a lot of armor due to your strength (just like you gain allres through int as a wizard) and can hit 8000 armor easily.

There are a lot of forum threads about good armor/allres values, as well as suggested EHP values for your respective goal. There is no "one size fits all" solution.

I'm not sure what these guys are talking about ^^ but from a mathematical perspective, 10 armor gives you the same bonus as 1 resist all at the same level of damage mitigation. 5000 armor is the same mitigation as 500. I think some of these guys are talking about which is better, which will be relative because there are diminishing returns on damage mitigation.

5000 armor OR 500 RA give you about 61% mitigation. If you add another half (7500/750) then you aren't looking at 90% mitigation, you're only looking at 70%. So while it's true that 10 armor = 1 ra at the same mitigation level what you should be doing is focusing on whichever number is lower, either armor/10 or RA.

There's another common misconception out there too that HP/Vit is the best EHP bonus. This is true if monsters are all hitting you for the same amount and those are the only monsters out there. If you are fighting keywardens or uber bosses, those guys hit you harder and while armor/ra scale with damage, HP does not. I've seen some people say HP is 100 to 1 RA, but that's simply false since the value of HP/Vit is absolutely relative to your armor, RA and amount of damage you're taking from the biggest enemies you face.

armor is way cheaper than allres at all times, balance is important but most of the time its smarter to get armor instead while having limited budget - everything depends on what are you up to, which class/spec are you running and what are your priorities

the more of both the better (obviously) but if you have to make a trade, I suggest using an EHP calculator to see what the most impact will truly look like. You can google one for diablo 3 that doesnt require any actual information other than stats you can read off of your character sheet and type onto the screen ( no giving any account information or anything like that )

I think some of these guys are talking about which is better, which will be relative because there are diminishing returns on damage mitigation.

There are no diminishing returns for survivability, either for armor or resistance. Use the search function to read the old threads on this topic. What makes many people talk about diminishing returns is that the damage reduction percentage (as shown on the in-game statistics page) isn't linear. However, if it was linear it would result in INCREASING returns (this was the case with resistances in Diablo 2). As soon as you understand EHP this becomes evident.

What others are talking about above is the fact that skills affect the relative value of armor and resist all. The 10:1 is from the strict mathematical formula as applied without any skill modifiers. If you are a barb using Tough as Nails (+25% armor), you will gain as much damage mitigation from 8 additional armor as from 1 additional resist all (8 x 1.25 = 10).

To sum everything up:
Without considering bonuses to armor and/or resistance from your skills, adding 10 additional armor or 1 additional resist all will increase your survivability by the same amount, regardless of your current armor and resistance.

To sum everything up:
Without considering bonuses to armor and/or resistance from your skills, adding 10 additional armor or 1 additional resist all will increase your survivability by the same amount, regardless of your current armor and resistance.

What the hell? This is simply not true as a generalization. 10 armor = 1 allres for EHP gain if both are currently at the same leve of mitigation. If you have 7000 armor and 200 allres, obviously adding 1 allres will give you much more EHP than adding 10 armor. See examples above or any EHP spreadsheet/tool (look at "EHP gained by stat" column).

So in terms of EHP, 10 armor = 1 all resist provided that you have a 10:1 ratio (roughly) on your char. Otherwise the lower stat will be somewhat more valuable.

However, there is a different thing to think about when you look at gear. The max bonus armor on gear is between 250 and 400, depending on the slot, while all resist caps at 80. So if you go for bonus armor, from one item you can get:
400 base armor + 400 bonus armor = 800 armor (= 80 all res equivalent)
while if you go for all resist you can get
400 base armor + 80 all resist (=120 all res equivalent, or 1200 armor)

The result is that on items where you can only have one defensive affix (e.g. rings, helms, gloves, amulets that require multiple DPS affixes to be good), it is always better to have high all resist.

Further, if you plan to reduce all resist in favor of armor, there's some form of DR kicking in. Say you have the option to go for 4k armor/400 all resist or 6k armor/200 all resist, we can compare the total damage reduction against a lvl 63 mob:
4k armor/400 ar: damage taken = (1-0.5594)*(1-0.5594) = 0.194
6k armor/200 ar: damage taken = 0.211

The different seems small, but we can look at EHP instead (neglecting dodge, block, and similar stuff), based off 40k health:
4k/400: 209k EHP
6k/200: 190k EHP

I'm using armor/(armor + 50*mlvl) and ar/(ar+5*mlvl) for the damage reduction. Results are rounded, but I do not propagate the rounding errors.

The math does indeed show that the ratio is 10:1 in EHP effectiveness when you're already at the correct ratio, but what does that actually mean? It is a guideline for what upgrades you should look for.

If you stack all resist, and have 900 RA, but only 4000 Armor, you would actually gain more EHP by trading out some RA for some +Armor. If you have 7000 Armor and 750 RA, they are close enough that gaining either will provide an EHP boost, although armor would be more effective.

Theorycrafting is important, but it doesn't occur in a vacuum. Eventually you have to use the math to decide what upgrades your character needs. That is how you should use the 10:1 ratio.

The math does indeed show that the ratio is 10:1 in EHP effectiveness when you're already at the correct ratio, but what does that actually mean? It is a guideline for what upgrades you should look for.

If you stack all resist, and have 900 RA, but only 4000 Armor, you would actually gain more EHP by trading out some RA for some +Armor. If you have 7000 Armor and 750 RA, they are close enough that gaining either will provide an EHP boost, although armor would be more effective.

Theorycrafting is important, but it doesn't occur in a vacuum. Eventually you have to use the math to decide what upgrades your character needs. That is how you should use the 10:1 ratio.

Most of what you said is about right, but especially the last bit ("theorycrafting is important, but it doesn't occur in a vacuum") is also right when it comes to the 10:1 rule.

10:1 is just a very very very rough goal, because of two reasons:

1) Like thundersteele said, the allres affix can go up to 80, while armor can not be higher than 397 (helm, chest, pants, amulet only) / 265 (all other slots) / weapon+OH zero. You have to keep this in mind: you can't simply switch 60 allres for 500 armor, because 500 bonus armor does not exist. Let alone switching 80 allres for 800 armor - would be nice, but it's impossible.
2) Wizards/WDs or barbarians will have 200-300 bonus allres or 2000-3000 bonus armor, respectively (through their main stat). Therefore, most wizards going for high EHP will just get a few pieces with 70-80 allres and already be at a good allres level, while it's almost impossible for wizards to go beyond 5000 armor without gimping themselves (e.g., giving up too many affix slots for armor). For barbarians the 10:1 rule might be more feasible, but many wizards end up having about a 6:1 ratio and are perfectly fine!